I think that GUI's with sharp corners look very unprofessional. Gui's with round corners make them feel more natural so I want to know how to make GUI's with round edged corners.
You would do something similar to Nine-slice. Nine-slice means that a frame is subdivided into 9 parts: 4 corners, 4 sides, and one center.
https://i.imgur.com/GwFSOvj.jpg
When you scale the gui horizontally, the top-side slice, bottom-side slice, and the center slice scale horizontally, whereas the rest just move along. A similar thing happens when you scale the frame vertically. What's important is that your 4 corners never scale (they maintain a square-aspect ratio).
Here's a plugin that should create 9-slice frames for you: http://www.roblox.com/Stravant-NineSlice-item?id=171907076